The present invention relates to a fabric and to a strip for reinforcement of canvasses having a plastic coating.
Canvasses or sail clothes having a plastic coating, e.g. on a textile tissue, are used as tent material and as material to cover and protect the cargo or loads on vehicles or containers. A number of requirements are put on these canvasses.
A first requirement that the canvasses must give a sufficient protection against vandalism and robbery. Here it is postulated that a proper canvass should at least delay the action of an opportunist thief who acts by means of a knife or cutter or by means of a pair of shears. The delay should last a number of minutes.
A second requirement is that canvasses must prevent the load from uncontrolled horizontal movements without tearing.
A third requirement is that initial cracks in canvasses must be prevented from growing.
A fourth requirement is that canvasses together with their reinforcement must have a weight which is as low as possible.
The prior art has already provided a solution which meets three of the above-mentioned requirements. Such a prior art canvass is reinforced by means of a woven fabric of stainless steel wires or cords.
Following drawbacks, however, are discovered with such prior art canvasses.
First of all such canvasses are very expensive due to the high price of the stainless steel fabric and to the expensive way of manufacturing such a canvass.
Secondly, the stainless steel fabric increases the weight of the canvass to an unacceptable degree so that the above-mentioned fourth requirement is not met.
And thirdly, the stainless steel fabric negatively influences the appearance or outlook of the canvass and as a result any publicity or brand names are no longer clearly pronounced on the canvass. Aramid reinforcements may provide a solution to the first and third drawback but remain expensive and they do not give a sufficient resistance against the action of a cutter or a knife.
It is a general object of the present invention to avoid the drawbacks of the prior art.
It is a first object of the present invention to provide for a low-cost reinforcement for canvasses.
It is a second object of the present invention to provide for a reinforcement for canvasses with an acceptable weight.
It is a third object of the present invention to provide for a reinforcement of canvasses which minimizes the influences on the appearance or outlook of canvasses.
According to one aspect of the present invention there is provided a fabric for reinforcement of canvasses having a plastic coating. The fabric comprises a warp and a weft which form meshes. These meshes have a maximum dimension ranging from 5 cm to 30 cm, preferably from 5 cm to 25 cm. Most preferably this maximum dimension is adapted to the kind of goods to be protected, but the most suitable dimension of these meshes is preferably about 7 cm to 15 cm, for example about 8 cm to 12 cm, in order to slow down the action of a thief which tries to penetrate his fist into such a mesh in order to take away goods. At least one of the warp or the weft (but preferably both the warp and the weft) is (are) formed by a strip which comprises a matrix of a thermoplastic material which is adherable to the plastic coating of the canvasses.
The strip further comprises two or more elongated metal members, preferably located parallel in the plane of the strip, in order to provide sufficient resistance against the cutting action of a knife or against the action of a pair of shears. The plurality of elongated metal members give to the strip the required strength and simultaneously enable the strip to remain thin and flexible. The strip has a cross-section with at least one flat side and an average thickness ranging from 0.50 mm to 3.0 mm, preferably ranging from 0.50 mm to 2.00 mm, and a cross-sectional width ranging from 3 mm to 25 mm, e.g. ranging from 5 mm to 25 mm. This flat cross-section enables the strip to remain thin whilst simultaneously providing a sufficient surface for adhesion between the canvass and the fabric.
The strips forming the warp may be connected to the strips forming the weft by means of an adhesive or by means of a welding technique where it is not necessary that the elongated metal members are welded to each other: it is sufficient that the connection is made by means of the thermoplastic material alone. The welding or at least contacting of one or more metal members of the warp to one or more metal members of the weft is, however, not excluded. This has the drawback that the welding is more expensive, but has the advantages that the final fabric is much stronger and that the fabric can be used as electrical circuits which may provide additional security.
According to one embodiment of the fabric the strips forming the weft lie above the strips forming the warp (or vice versa). Adhering such a fabric to a canvass leads to a canvass which is relatively flexible in the direction of the strips (warp or weft) that are adhered to the canvass over their complete length and relatively stiff in the direction of the strips (weft or warp) that are not adhered to the canvass at the points of crossing with the other strips.
According to another embodiment of the fabric the strips forming the weft lie alternatingly under and above the strips forming the warp. Adhering such a fabric to a canvass leads to a canvass which is equally flexible in both the warp and weft direction.
The functionality and flexibility of a fabric may also be influenced by the type of metal members used to reinforce the strips. High carbon steel cords (carbon content above 0.7%) have the advantage of being relatively flexible, of having a high strength and of adhering mechanically well to the matrix material of the strip due to their undulated outer surface. They provide a good remedy against the action of a knife or a cutter. The steel cord may have a high elongation at break, e.g. an elongation at break exceeding 5%, so that much energy can be absorbed before the steel cord breaks. In a particular embodiment of the invention the steel cord has two or more twist angles which differ substantially from each other.
Substantially differing twisting angles has the advantage of offering an improved resistance against stabs. Preferably the steel cord is free of residual torsions and of other tensions in order to avoid that the steel cord would become wild when the strip is welded under heat to the canvass.
The inventors have experienced, however, that metal members which are more ductile than high carbon steel cords provide an improved resistance against the action of a pair of shears or a pair of scissors and that this resistance is even increased if the ductile member does not adhere to the matrix material. Examples of ductile members are a copper wire, which has the advantage of being very suitable for use in an electrical circuit or a low carbon steel wire (carbon content below 0.4%) which can be thermally treated to further increase its ductility. The steel wire can be a round steel wire or a flat steel wire. In comparison with steel cords, the wires have a much smoother surface and adhere mechanically not so well to the matrix material. With respect to the resistance against the action of a pair of shears, this has been experienced more as an advantage than as a drawback. The copper or steel wires are, however, less flexible than steel cords but for equal strengths a steel wire is less expensive but less flexible.
Canvasses for trucks may be divided into two main categories: canvasses of the curtain type and canvasses of the roll up type. Canvasses of the curtain type are slidingly suspended on horizontal rails and can be horizontally slid to one side to open the canvass. Canvasses of the curtain type require flexibility in the horizontal direction. Canvasses of the roll up type can be rolled up vertically to open the canvass. Canvasses of the roll up type require a flexibility in the vertical direction. Fabrics according to the invention may be realized so that there are strips reinforced with flexible metal members such as steel cords in one direction (the horizontal for canvasses of the curtain type and the vertical for canvasses of the roll up type) and strips reinforced with ductile but less flexible metal members such as metal wires in the other direction.
Various types of metal members can also be combined in a single strip so that the single strip offers both a good resistance against the action of a knife or a cutter and a good resistance against the action of a pair of shears.
According to a second aspect of the present invention, there is provided a strip for reinforcement of canvasses having a plastic coating. The strip comprises a matrix of a thermoplastic material which is adherable to the plastic coating of the canvasses. The strip further comprises two or more elongated metal members. The strip has a cross-section with at least one flat side and with an average thickness ranging from 0.50 mm to 3.0 mm (preferably to 2.0 mm) and a cross-sectional width ranging from 3 mm to 25 mm.
The thermoplastic material is preferably of the same nature or preferably has a similar composition as the plastic coating of the canvass. Canvasses are usually made of a flexible polyvinylchloride but may also be made of a flexible polypropylene or polyethylene or even of polyester.
Preferably four to twenty metal members reinforce one single strip. In order to obtain a strip which is flat and remains flat, single-twisted cords may function as metal members where a Z-twisted cord alternates with an S-twisted cord and vice versa along the width of the cross-section of the strip. The breaking load of all the metal members in one single strip is preferably higher than 2000 Newton.